Electrocomponent Science and Technology
1977, Vol. 3, pp. 217-224
(C)
Gordon and Breach Science Publishers Ltd., 1977
Printed in Great Britain
ELECTROFORMING, SWITCHING AND MEMORY
EFFECTS IN OXIDE THIN FILMS
D. P. OXLEY
Department of Physics, Leicester Polytechnic, Leicester, England
(Received December 14, 19 76)
Experiments on electroforming of Metal-Oxide-Metal thin film sandwiches which have been electroformed to
exhibit voltage-controlled negative resistance are summarized and an outline of recent evidence in favour of localized
or fihmentary conduction is given.
A similar review is given of the experiments on oxide sandwich structures which have been formed to exhibit
current-controlled negative resistance or threshold switching and memory switching. Current theories are reviewed
briefly. Finally oxide memory devices are compared with those based upon the chalcogenide glasses.
1.
INTRODUCTION
2) A large class of metal-amorphous oxide-metal
thin film sandwiches could be ’formed’ to show an
enhanced conductivity in a vacuum and a low
frequency VCNR and analog memories. Many
workers refer to this process as electroforming.
3) Other forming processes can lead to CCNR and
threshold or memory switching.
4) Some oxides can undergo both types of forming.
5) That the models advanced to explain these
effects divide into two main groups consisting of
schemes envisaging a relatively homogenous modification of the insulator during each type of forming or
those envisaging localized or filamentary conduction
in the formed state.
A wide range of amorphous and crystalline oxide thin
films when sandwiched between suitably selected
electrodes (usually metal) can be processed so that
they exhibit a wide range of interesting electrical
properties. Such sandwiches can be made to exhibit
voltage-controlled negative resistance (VCN R),
current-controlled negative resistance (CCNR),
threshold switching, analog memory switching,
bistable memory switching and other potentially
useful electrical effects. These properties have been
reviewed by Dearnaley et al and a later bibliography
listed by Agarwal 2 includes some references to
switching phenomena. Further papers have appeared
since these articles were written and the literature of
these effects is now quite extensive. The present paper
will give a brief, but hopefully representative, review
of this literature summarizing the reported practical
significance and compare these switching and memory
phenomena with those found in chalcogenide glasses.
2.
The processes leading to the classes of behaviour
in
(2) and (3) will each be discussed in more detail
and the properties of the formed states reviewed.
3.
ELECTROFORMING LEADING TO VCNR
Devices to be electroformed usually consist of a
central insulating layer of thickness in the range
20 nm to about 1000 nm, a thickness of 100 nm
being typical. The insulator is frequently an amorphous oxide layer prepared either by vacuum
deposition or anodic oxidation. The selection for the
conducting electrodes to complete the sandwich is
limited for the anode (the electrode held at a positive
potential during electroforming and subsequent
operation) to specific materials;the choice of cathode
being less critical. The anode is usually chosen by
FORMING
The review by Dearnaley et al summarized a significant body of experimental evidence to show that:
1) Many metal-amorphous or crystalline oxidemetal thin film sandwich structures could have their
electrical properties changed in a radical and irreversible manner by the application of a bias to the
electrodes. This process has become known as
’forming’.
217
D.P. O XLEY
218
experiment although the balance of evidence would
indicate that inert metal anodes assist in obtaining the
electroformed state. Electroforming is carried out in a
vacuum. The following results (Figure 1) from the
early paper by Verderber et al 3 show the difference
between the formed and unformed state for an
Au-SiOx-A1 sandwich where the gold electrode is
positively biased in a vacuum of better than 10 -2
torr. When the bias is first applied the current is
commensurate with that expected on the basis of the
conductivity of the unformed insulator, i.e. small. At
a critical bias, VF, the current increases quite rapidly
and irreversibly. Verderber et al report that at a given
temperature this voltage VF is reasonably reproducible for nominally identical structures and that it does
not depend strongly on the thickness of the insulator;
this indicates that electroforming is not simply related
to the macroscopic average field. Later work indicated that VF varied somewhat with insulator and
electrode composition. As the bias is cycled at a
constant low frequency between 0 and V, the
forming progresses until the I, V characteristic
becomes reasonably reproducible (Figure 2, curve
O
o
1(5 4,
O
/
SiO=40nm
A
V
O
I, V characteristic of formed device and an
analog memory state (schematic diagram).
FIGURE 2
OABC). The current in the pronounced region of
VCNR is noisy. The VCNR is destroyed if the bias is
cycled whilst the device is exposed to air (oxygen
seems to be the important gas 3) or if the temperature
is reduced to about 100 K4 or if the device is
operated under reverse bias. After such treatments
the average conductivity is low and the/, V characteristic non-ohmic. These effects can be reversed.
Although prolonged operation in air usually causes
degradation of the device, s’ 7
The electroformed structure can be used as an
analog memory as described by Simons and
Verderber. 4 They applied a bias Va > Vp, the voltage
for maximum current, to put the device into the
region of VCNR. This bias was then removed rapidly,
in about 0.1 ms. During this rapid removal, the downward sweep of the/, V locus does not show VCNR
(Figure 2). After this process, for a bias below a value
Vr, the I, V characteristic will be that for the
previous downward sweep. Simmons and Verderber4
claim that this state appeared to be stable indefinitely
provided that the bias did not exceed Vr and that the
memory state was reversible, and that the original
low impedance memory state OA could be regenerated by applying a voltage slightly in excess of Vr.
They reported that the sample could exist in any one
of a ’continuum’ of memory states corresponding to
rapid removal of the bias within the VCNR region.
The accepted functional form of these characteristics
is that given by Simmons and Verderber 4 viz:
I K(Va)Sinh k(Va)V
FIGURE
From Verderber et al. showing change in
current upon electroforming A1-SiOx-Au.
where both K(Va) and k(Va) decrease in magnitude
OXIDE THIN FILMS
with increasing Va. Typically for Va 6 V,
K(Va) 1.1 x 10 -3 A and k(Va) =0.5 V -1
4.
For a given memory state and a bias V < VT then
the I, V characteristics are stable as the temperature is
decreased with a slight quadratic dependence in the
mean conductance o. For applied voltages above V
but less than VT-, Simmons and Verderber.4 suggest
that:
6o(1 + aT)
a
with
o , 10-6K -2
After electroforming devices usually emit electrons
and exhibit electroluminescence. Figure 3, after
Collins and Gould 6 shows representative results for
the variation in emission current with device bias.
Emmer 7 has reported that all of the electroformed
Ic/mA
-Ioq
40
3O
z
rIC
(le/A)
Ie
/
I
/I
IO
20
12
bis/V
FIGURE 3 From Collins and Gould showing variation in
circuhting current c and emission current with device bias
for
A1-SiO-Au structure.
A1-A1203 oAu structures examined exhibited varying
amounts of anode damage which could enhance
electron emission. Attempts to use this emission
process to infer hot-electron mean-free paths or
attenuation lengths have recently been shown to be
in error, s and although this analysis was criticized 9
the authors have replied. o
CONDUCTION AND MEMORY STATES
AFTER ELECTROFORMING
Work performed since the review by Dearnaley et al
has seriously reduced the credibility of models
involving homogenous forming. Single shot pulsed
bias studies have added weight to filamentary
approaches. The interpretation of experimental data
cited as indicating the existence of local regions of
high electric fields near the anode or cathode in
electroformed structures has been criticized on simple
ftmdamental grounds, 12,1 3 which seriously weakens
many band approaches. Equally disconcerting from a
band model viewpoint are the experiments of
Emmer 7 giving evidence for localized electron
emission.
It would thus appear that the conduction in
electroformed structures is localized and that the
conduction, memory phenomena, electron emission,
electroluminescence and current noise are related to
the processes envisaged by Dearnaley et al 14 in terms
of voltage controlled selective rupture and regrowth
of filamentary conduction paths. Dearnaley et al 14
suggest that the number of such filaments would be
about 5 x 10 4 mm -2 with diameters of the order of
nm. The detailed nature of the filaments was not
specified.
Recently Bischoff and Pagnia have made gold
island structures and report electroforming and d.c.
and a.c. I-V characteristics which resemble those in
electroformed diodes. They claim that their structures
are suitable for electron microscope investigations of
the forming and switching process and provide an
excellent tool to clarify the fundamental behaviour
of electroformed diodes.
5.
Applied
219
BISTABLE SWITCHING AND CCNR
The detailed forming procedures used to obtain CCNR
and bistable switching vary rather more than the
procedures used to produce VCNR. Equally the device
characteristics after forming are more varied than for
devices showing VCNR. Despite these variations many
of these forming processes have a common advantage
in that forming need not take place in a vacuum. The
formed devices are potentially more useful as their
properties can be stable under bias in air. Dearnaley
et al have reviewed the earlier work in this area to
show that some oxides can separately exhibit both
VCNR and CCNR after suitable forming. This review
cites the early reports of Chopra on thermally grown
50 nm oxide layers of Nb, Ta, Ti. Metal-metal oxide-
2 20
D.P. OXLEY
metal sandwiches exhibiting rectifying characteristics
are formed under reverse bias, by increasing the
reverse current to above mA mm -2 to transform
the/, V characteristic to a symmetrical shape
exhibiting CCNR followed by a region where the
dynamic impedance approaches zero (Figure 4). The
holding voltage to sustain this state is conveniently
small and is independent of oxide thickness and
electrode material (as in Ovshinsky 16 threshold
switches for metal electrodes); the CCNR persists at
frequencies of up to MHz, in contrast with the
VCNR in electroformed structures which is limited
to lower ( 10 kHz) frequencies, and on this basis it
was argued that the mechanism leading to such CCNR
was electronic. Chopra 7 in his original paper
emphasised the reproducibility of his devices
compared with those exhibiting VCNR and
established their relative insensitivity to ambient air.
The variation in forming conditions leading to
CCNR in oxide layers is illustrated by comparison of
the previous method of Chopra 17 with that followed
by Hickmott and Hiatt 18 for their Nb-Nb2 O2-Bi
structures. They developed empirically a complex
breakdown (forming) procedure using both pulsed
and steady but limited currents to produce devices
with stable characteristics (Figure 5). The final device
has two distinct non-ohmic states with high and low
average values of conductance. Switching between
these states was described but in view of the reported
sporadic nature of the switching, the device degradation under sinusoidal drive at 100 kHz, and the gener
general absence of long term stability in any state,
FIGURE 4 Showing CCNR in Nb-Nb oxide-Au structure.
After Chopra 17.
0.2
0.4 0’8
Bi Positive
FIGURE 5 Showing memory states for
structure. After Hickmott and Hiatt’
s.
Nb-Nb O s-Bi
they are of little direct interest for memory application in reported form. It is of interest to note however
that Hickmott and Hiatt 8 confirmed earlier
reports 19 that the forming results from the modification of a very small region of the oxide of less than
10 -3 mm 2 in contrast with Chopra 7 who suggested
that the whole structure was active in his devices.
Further variations in forming methods and characteristics of the formed devices are reported in the work
of Basavaiah and Park2 o for Nb-Nb20 s-Bi junctions.
They used a simple forming procedure, applying a
bias of +30 V via a 50 KYZ resistor to the bismuth
electrode to reduce the resistance from about 107fZ
to about 5 K. Memory switching between two states
with a binary signal ratio of 8:1 for more than 106
operations was reported. Typical switching times were
1/as and 20/as from the high to low and low to high
resistance states respectively. Conduction in the
formed state was associated with the modification of
a small ("2/a2) region in the oxide layer which was
observed on the top bismuth film. It was suggested
that the conduction in the high resistance state was
related to a Schottky mechanism whereas that in the
low conductivity state was not identified. More
recently Lalevic et al21 reported a further variation;
this work used Nb-Nb2-O s-Nb structures with Nb
doped amorphous Nb20s films. Forming was initiated
using a bias of 14 V and a current density of
1.34 mA mm -2 During this process the resistivity
changed from 1.4 x 101 o f’tm to 2.9 x 108 Zm. At
this state a reduced bias of 1.25 V at a current of
3.0 mA was necessary to bring the device to a final
state with a resistivity of 5 x 107 m. Repetitive
switching between bistable states was then claimed
D. P. OXLEY
221
Z
Z
222
OXIDE THIN FILMS
r. o
0
0
OXIDE THIN FILMS
with the switching state characterized by a CCNR.
The observed delay time in switching was >1 ms
which was used to argue a double-injection model
of the switching mechanism. They also claimed that
their devices could switch in times and sustain power
levels which compared favourably with amorphous
chalcogenide thin film structures.
The earlier work by Chopra included reports of
high frequency CCNR in formed Ti-TiO2-Au
structures. The later data obtained by Soukup 2 on
Ti-TiO2 -Au structures who reported CCNR only under
slowly varying bias would appear to be in conflict.
Recently Taylor and Lalevic2 a have obtained stable
oscillations in the 1-3 MHz range using the CCNR in a
Ti-TiO2-Cr device. These variations may be associated
with electrode composition or forming procedures.
Taylor and Lalevic2 suggest that the measured/, V
characteristics for their devices are in agreement with
the theory of filamentary double injection of space
charge limited current.
Switching has also been reported by other workers,
some using other oxides (Table 1) and it can be seen
that a clear theoretical understanding has yet to
emerge. For memory switching the balance of
evidence perhaps points to filamentary conduction in
the on state whilst for the threshold switches the
picture is less clear.
6.
223
This was achieved using velocity saturation effects in
amorphous/crystalline Si heterojunctions and
transient on state conductivity measurements. They
emphasize that at a high current density in a 1-2
layer of chalcogenide glass the ’filament’ diameter will
be 50 m provided that pore saturation is not
evident, hence their comment "Note that the filament
has the appearance of a pancake rather than a
long narrow channel". Such filament diameters lead
to relatively low (< 100C) pore temperatures and
may assist in long term stability.
Reference to Table shows that oxide sandwich
structures can exhibit threshold and memory switching. However, in view of the present limited use of
chalcogenide switches which, most importantly are
of established compatibility with silicon technology,
are relatively more reliable, and also better understood, it seems unlikely that oxide switches will be
sold in the next decades. It will be interesting from a
fundamental viewpoint to continue to examine
switching in oxides as a clear theoretical position has
yet to emerge. For the development of any possible
future application of oxide switches it is perhaps best
to attempt to understand the memory switches. These
can be used, like chalcogenide switches, to perform
read-mostly functions not accessible by silicon
technology alone, a
COMPARISON WITH OVSHINSKY DEVICES
Ovshinskyl 6 switches, both memory and threshold,
are based upon layers of chalcogenide glass, sandwiched between suitable electrodes. The mechanism
of operation of the memory switch has for some
time been understood to be associated with the
growth of a highly conducting polycrystalline
filament during the lock-on process whilst
revitrification of the polycrystalline region occurs
during the reset process to a lower conductance
state. Etching techniques have verified these ideas
in detail and led to improved device performance.
Threshold switching, using rather more stable,
chalcogenides, has only recently been understood in
detail. Petersen and Adler 34 have cited excellent
evidence to show that in the case of chalcogenide
threshold switches the conduction in the on state is
that for a semi-conductor whose electronic band
structure is essentially unchanged from the off state,
but in which a concentration of about 10 2 o electrons
m -3 yields the high conductivity. They have refined
the accepted filamentary model of conduction by
measuring the filament size as a function of current.
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(1974).
3. R. R. Verderber, J. G. Simmons and B. Eales, "Forming
processes in evaporated SiO thin films", Phil. Mag., 16,
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4. J.G. Simmons and R. R. Verderber, "New conduction
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7. I. Emmer, "Conducting filaments and voltage-controlled
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D.P. OXLEY
224
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